Model Checking with SPIN Modeling and Verification with SPIN

1. Model Checking with SPINModeling and Verification with SPIN Wishnu Prasetya wishnu@cs.uu.nl www.cs.uu.nl/docs/vakken/pv

2. Overview • Architecture & a bit more about SPIN • SPIN’s modeling language • Examples of models in SPIN • Acknowledgement: some slides are taken and adapted from Theo Ruys’s SPIN Tutorials.

3. Spin and Promela • SPIN = Simple Promela Interpreter • Promela =Process Meta Language • Is a modelling language! (not a language to build an application) • Strong features : • Powerful constructs to synchronize concurrent processes • Cutting edge model checking technology • Simulation to support analysis (of the models)

4. SPIN • Concurrency is a hot area again, now that we all use multi-core CPUs. • Other applications: • AnWeb: a system for automatic support to web application verification, Di Sciascio et al, in 14th conf. on Soft. Eng. and knowledge eng., 2002. • Privacy and Contextual Integrity: Framework and Applications, Barth et al, in IEEE Symposium on Security and Privacy, 2006.

5. Frontend XSpin

6. •deadlocks •safetyproperties •livenessproperties spin command line tool random guided interactive ϕ LTL Translater Simulator Promela model M ispin spin Verifier Generator editingwindow simulationoptions verificationoptions MSCsimulation window C program pan.* counter example false checker pan.exe (X)SPIN Architecture

7. System, process, and action. • A system in SPIN consists of a set of interacting and concurrent processes. • Each process is sequential, but possibly non-deterministic. • Each process is built from atomic actions (transition). • Concurrent execution is modeled by interleaving. • Fairness can be impossed.

8. Interleaving model of concurrency • Consider (with pseudo notation):Assume each arrow is atomic. • An execution of P||Q abstractly proceeds as one of these paths : x++ x++ P : print x Q : (note the interleaving)

9. Degree of atomicity • Whether it is reasonable to model a statement as ‘atomic’, depends on your situation. • x++ usually no problem • x>0  y:=x ok, if we can lock both x and y • 0S  found:=true ....?

10. Example bytex = 1 ; active proctype P1() { x++ ; assert (x==2) ; } active proctypeP2() { x-- ; } (using a global variable to interact)

11. Data types • Bit 0,1 • Bool true, false • Byte 0..255 • Short -215 .. 215-1 • Int -231 .. 231-1 • Pid 0..255 • Mtype 0..255 // user-def. enumeration • Chan 0..255 • One dimensional array • Record

12. What you don’t have… • No sophisticated data types • No methods ; you have macro • There are only 2levels of scope: • global var (visible in the entire sys) • local var (visible only to the process that contains the declaration) • there is no inner blocks

13. (Enabledness) Expression • This process has 3 atomic actions. • The action “y==0” • only enabled in a state where the expression is true • it can only be executed when it is enabled; the effect is skip • so, as long as it is disabled, the process will block • if it is not enabled in the current state, a transition in another process may make it enabled in the next state. • even if it is enabled in the current state, there is no guarantee the action will be selected for execution; but there is a way in SPIN to impose fairness. active proctype P { x++ ; (y==0) ; x-- }

14. Example • Use it to synchronize between processes : • // both will terminate, but forcing Q to finish last byte x=0 , y=0 active proctype P { x++ ; (y>0) ; x-- } active proctype Q { (x>0) ; y++ ; (x==0) ; y-- }

15. Multiprogramming is tricky…. • E.g. one or more processes can become stuck (deadlocked) : (6 potential executions…) byte x=0 , y=0active proctype P { x++ ; (y>0) ; x-- ; (y==0) }active proctype Q { y++ ; (x>0) ; (x==0) ; y-- }

16. Processes can also synchronize with channels chan c = [3] of {byte} ; active proctype producer() { do :: c ! 0 od } active proctype consumer() { do :: c ? x od }

17. Channels mtype = { DATA, ack } • for exchanging messages between processes • finite sized and asynchronously, unless you set it to size 0  synchronous channel • Syntax : c ! 0 sending over channel c; blocking if c is full c ? x receives from c, transfer it to x; blocking if c is empty d ? DATA, b, y match and receives • There are some more exotic channel operations : checking empty/full, testing head-value, copying instead of receiving, sorted send, random receive ...  check out the Manual chan c = [0] of {bit}; chan d = [2] of {mtype, bit, byte}; chan e[2] = [1] of {bit};

18. Conditional if :: stmt1 :: … :: else -> … fi if :: stmt1 :: … :: stmtn fi • The alternatives do not have to be atomic! • The first action in an alternative acts as its “guard”, which determines if the alternative is enabled on a given state. • Non-deterministically choose one enabled alternatives. • If there is none, the entire IF blocks. • “else” is a special expressionthat is enabledif all otheralternativesblock.

19. loop : do-statement • Non-deterministic, as in IF • If no alternative is enabled, the entire loop blocks. • Loop on forever, as long as there are enabled alternatives when the block cycle back. • To exit you have explicitly do a break. do :: stmt1 :: … :: stmtn od

20. Non-determinism can be useful for modeling active proctypeconsumer() { do :: c ? x ; :: c ? x ; x=corrupted ; // to model occasional corrupted data od}

21. Exiting a loop do:: { (i>0) ; i-- }:: { (i==0) ; break }do do :: (i>0) i-- :: (i==0) breakdo do :: { i-- ; (i>0) } :: breakdo

22. Label and jump • Labels can also be useful in specification, e.g. <> P@L0 • Referring to labels as above goes actually via a mechanism called “remote reference”, which can also be used to inspect the value of local variables for the purpose of specification. L0: (x==0) ; if :: … goto L0 ;:: …fi

23. Expressing local correctness with assertions active proctype P …active proctype Q { …; assert (x==0 && y==0) } (here it implies that when Q terminates, x and y should be 0)

24. But we can also express global invariant! • Thanks to built-in non-determinism in the interleaving semantics, we can also use assertion to specify a global invariant !// implying that at any time during the run x is either 0 or 1 byte x=0 , y=0 active proctypeP { x++ ; (y>0) ; x-- }active proctypeQ { (x>0) ; y++ ; (x==0) ; y--} active proctypeMonitor{ assert ((x==0 || x==1)) }

25. Deadlock checking • When a system comes to a state where it has no enabled transition, but one of its processes is not in its terminal (end) state: • Deadlocked, will be reported by SPIN • But sometimes you want to model that this is ok  suppress it via the invalid-endstate option. • The terminal state of a process P is by default just P’s textual end of code. • You can specify additional terminal states by using end-label: • Of the form “end_1” , “end_blabla” etc

26. Expressing progress requirement • We can mark some states as progress states • Using “progress*” labels • Any infinite execution must pass through at least one progress label infinitely many often; else violation. • We can ask SPIN (with an option) to verify no such violation exists ( non-progress cycles option).

27. Dining philosophers • N philosophers • Each process: • grab left and right fork simultaneously • eat... • release forks • think................ then go back to 1

28. The processes in Promela atomic { ..... } #define N 4 byte fork[N] ; bool eating[N] ; proctype P(byte i) { do :: (fork[i] == N && fork[(i + 1) % N] == N) -> { fork[i] = i fork[(i + 1) % N] = i ; eating[i] = 1 ;// eat ... eating[i] = 0 ; fork[i] = N ; fork [(i + 1) % N]= N } od } • Why use bytes ? • Should we enable the default end-state checking? • How to instantiate the P(i)’s ? • Ehm... this is not correct !

29. Creating processes and init { … } init { byte i ; ... // initialize forks i = 0 ; do :: i<N -> { run P(i) ; i++ ; } :: i>=N -> break ; od } Put this in atomic { … } ; Be aware of what it means! What if we want to show that the algorithm is still correct for any initial value of forks, as long as you have at least one pair of forks free at the beginning, and hat forks are only taken in pairs?

30. Using non-determinism to quantify over your data init { // initializing the array x byte i = 0 ;byte v ; do :: i>=N -> break ; :: { if :: v = N :: v = i fi ; fork[i]=v ; fork[(i+1)%N]=v ; i++ ; } od ; …. // now create the processes as in the previous slide }

31. How to express the specification? proctype P(byte i) { do :: { atomic {(fork[i] == N && fork[(i + 1) % N] == N) ; fork[i] = i ; fork[(i + 1) % N] = i } ; eating[i] = 1 ; eating[i] = 0 ; fork[i] = N ; fork [(i + 1) % N]= N } od } assert (fork[i] == i && fork[(i+1)%N]== i)

32. Using a “monitor” process activeproctype monitor() { byte i ; i = 0 ; do :: i>=N -> break ; :: i<N -> { assert(!eating[i] || (fork[i]==i && fork[i+1%N]==i)) ; i++ ; } od } But we still can’t express that if a process is “hungry”, it will eventually eat. In this particular problem, we can still express it using progress labels. For more general temporal specification, we will look at the use of LTL formulas.

33. Example: Alternating bit protocol • imperfect “connections”, but corrupted data can be detected (e.g. with checksum etc). • Possible solution: send data, wait for a positive acknowledgement before sending the next one. Just 1 bit is needed for the ack, hence the “bit” in the name. sender receiver

34. You can think of several ways to work it out... • A note on reliable full-duplex transmission over half-duplex links, K. A. Bartlett, R. A. Scantlebury, P. T. Wilkinson, Communications of the ACM, Vol 12, 1969. • NPL Protocol • M<2 Protocol (we’ll discuss this one) • For more, check out: http://spinroot.com/spin/Man/Exercises.htmle.g. Go-Back-N Sliding Window Protocol

35. M<2 Protocol, Sender part Fetch a new data. 1 !1,data ?1 !0,data// Sender wants to resend 2 4 ? error !1,data ? 0 // Receiver wants Sender to resend 3 State 1 is the starting state, and its accepting state in the sense when the sender is in this state, it assumes the last data package it sent has been successfully received by the receiver, and so it fetches a new data package to send.

36. M<2 Protocol, Receiver part 1 !0 // request Sender to resend ? error !1 2 4 ?0,rd // Sender wants Receiver to resend ?1,rd !1 3

37. Scenario: 1x error, corrected 1 Fetch a new data. !0 // request Sender to resend ? error 1 !1 !1,data ?1 2 4 !0,data // Sender wants to resend ?0,rd // Sender wants R to resend 2 4 ?1,rd !1 ? error 3 !1,data ?0 // Receiver wants S to resend 3 Though each automaton is simple, the combined (and concrete) behavior is quite complex;  100 states in my (abstract) SPIN model (there are more explicit states, if we take the “data” into account).

38. Modeling in Promela chan S2R = [BufSize] of { bit, byte } ; chan R2S = [BufSize] of { bit } ; proctype Sender (chan in, chan out) { … }proctype Receiver(chan in, chan out) { … } init { run Sender(R2S, S2R) ; run Receiver(S2R, R2S) }

39. Modelling in SPIN proctype Receiver(chan in, out) { show byte rd ; /* received data */ show bit cbit ; /* control bit */ do :: in ? cbit, rd ; if :: (cbit == 1) -> out!1 :: (cbit == 0) -> out!1 :: printf("MSC: ERROR1\n") ; out!0 fi od } So, how big the channels should be? Is 0 good enough ?

40. A different style, with “goto” proctype Sender(chan in, out) { show byte data ; /* message data */ show bit cbit ; /* received control bit */ S1: data = (data+1) % MAX ; out!1,data ; goto S2; S2: in ? cbit ; if :: (cbit == 1) -> goto S1 :: (cbit == 0) -> goto S3 :: printf("MSC: AERROR1\n") -> goto S4 fi ; S3: out!1,data ; goto S2 ; S4: out!0,data ; goto S2 ; }

41. Specification, with assertions? • This time, not possible with assertions (at least not without the help of ‘something else’). • In LTL (to be discussed later), we can try something along this line : • But this still does not quite express the above. Each data package, if accepted by the receiver, is accepted exactly once! (Receiver@S3 (Receiver@rd == Sender@data))

42. Specification, using shadow variables • Extend the model with ‘shadow variables’ • Are used purely for expressing specifications • Must not influence the original behavior • In our case: • exploit that sender generates new data by data+1 • introduce a shadow variable “last”  previously accepted data • Impose this assertion on the acceptance state (of Receiver): Each data package, if accepted by the receiver, is accepted exactly once! current data to be accepted = last + 1

43. Extending the model proctype Receiver(chan in, out) { show byte rd ; /* received data */ show bit cbit ; /* control bit */ do :: in?cbit,rd ; progress: if :: (cbit == 1) -> out!1 :: (cbit == 0) -> out!1 :: printf("MSC: ERROR1\n") ; out!0 fi od } show byte last assert (rd == (last+1) % MAX) ; last = rd ; This is the Receiver’s accepting state S3

44. 2x successive errors 1 Fetch a new data. !0 // request Sender to resend ? error 1 !1 !1,data ?1 2 4 !0,data // Sender wants to resend ?0,rd // Sender wants R to resend 2 4 ?1,rd !1 ? error 3 !1,data ?0 // Receiver wants S to resend 3 Ouch…

45. Ok... but suppose we still want to verify these: But, if error does not occur twice successively then: every pck sent, if accepted, is accepted exactly once. If no error occur, every data sent will eventually be accepted. The first can be expressed simply by constraining the model, namely how it simulates error. The 2nd one can’t be expressed with just assertions and shadow variables. Alternative: LTL.

46. More on Promela

47. Exception/Escape • S unless E • Statement! Not to be confused with LTL “unless”. • If E ever becomes enabled during the execution of S, then S is aborted and the execution continues with E.More precisely… check manual.

48. Predefined variables in Promela • _pid (local var) current process’ instantiation number • _nr_pr the number of active processes • np_ true when the model is not in a “progress state” • _last the pid of process that executed last • else trueif no statement in the current process is executable • timeout true if no statement in the system is executable …

49. Timeout • timeout becomes executable if there is no other process the system is executable/enabled • so, it models a global timeout • useful as a mechanism to avoid deadlock • beware of statements that are always executable. do :: c ? x  ... :: timeout break od